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I have a Supermicro server with an external disk array connected using SAS. Assuming I have already shut down the server properly, is there anything I should do before I can power down the disk array by pressing the power button for four seconds?

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up vote 6 down vote accepted

Probably not. Once the operating system has written out its caches, closed the journal, and unset the dirty bit, as applicable for the filesystem in use, things are pretty much done.

If the external disk array isn't caching, this is definitely true and you can just cut power. If it is caching, it would normally provide its own mechanism for clean shutdown. Either way, if this is the documented way to power it off, and the thing that was using it is off, it's the right thing to do.

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One would assume that the disk array will begin flushing its caches when the power button is initially pressed; four seconds would be an adequate length of time to flush the cache in most cases. – Jules Aug 10 '14 at 7:37
One would so assume and hope, yes. – Falcon Momot Aug 10 '14 at 18:09

Once the server is halted, there's no need for you to do anything else.

Depending on the external enclosure, you'll want to make sure it's powered on and the drives spun-up before you start the server... so the reverse of of the shutdown process.

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Both previous answers are correct, I'd only like to emphasize one point - if the controller on the array is battery backed, you can shutdown immediately. Otherwise - you must "wait" for the controller's cache to flush to disks. This "wait" IIRC is mostly in the realm of 5 seconds at most, and probably just a few milliseconds in normal circumstances.

In any event, you should disable cache on the disks - this cache serves no purpose once you have controller cache, and in fact increases the risk for data loss/corruption in case of power loss.

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